Question about a C6

[adyszel]

I recently bought my first 6.9 f-250 (1985). Its got the C6 automatic tranny and 2WD. Paid 1000 bucks for it with about 212k. Not much rust, but somebody poorly spray-painted it black from its original red color. Looked like somebody at some point had installed a factory reman'ed DB2 and possibly new injectors. Two adjacent cylinders (6, 8) were running at low compression and missed (white smoke) when engine temp was cold. Right now I'm finishing up replacing the head gaskets, which should cure that issue.

My next task is to figure out what to do with the tranny. Its the C6 and shifts pretty decent throughout the two lower gears. But when i get to the high gear, if i do anything more then a slight press on the accelerator after it shifts up it immediately kicks back down to the lower gear. If I baby it by not throttling it too much I can get it to settle in the gear without downshifting. Does anyone have any tips? I was thinking it might be a matter of tuning the kickdown linkage but wasn't sure. I know that the vacuum can also play a role...

 

[suv7734]

Condition of vacuum pump (needs ~20"), the condition and setting on the VRV (valve on the driver's side of the injection pump) and the condition and setting of the vacuum modulator (at the tranny) all play a part. It sounds like it may just need some adjustment/troubleshooting. The kick down has adjustment as well but for troubleshooting I would just disconnect it and make sure the rest is working OK. I completely removed my kickdown and can still get it to produce the symptoms you've described if adjustments etc. are not right.

 

[adyszel]

Thanks for the info! I'll take the kickdown lever off and see if it persists. If it does, I'll get my vacuum gauge on the pump and check for 20 " Hg. How difficult is it to troubleshoot the VRV/tranny regulator?

 

[Diesel JD]

Its a pain! You really need a vaccum pump and a gauge and the right specs to do it correctly. You can usually get it close enough by the seat of your pants to work pretty decent and not hurt anything though. VRVs do go bad with some regularity as do vaccum pumps. I suggest you be very very careful if you monkey around with the vaccum modulator or VRV. It sounds like you have a pretty healthy C6 that just needs some slight adjustments. It can easily become a burnt C6 if you give it too much vaccum, but that's about the only way to kill one..that or run it completely out of fluid.

 

[Jake S.]

That would also be a misadjusted/bent kickdown rod. When I had mine it seemed that vrv was for upshifts and kickdown controlled downshifts.

 

[Diesel JD]

Yeah just take the kickdown rod loose at the transmission and see where you are after that. You don't really need the kickdown as long as you know to downshift when you are heavily loaded and lugging the motor or something of that nature.